1. Field of the Invention
The present invention relates to a method for detecting a moving object (a moving celestial object such as a man-made celestial object, space debris, an asteroid, a comet and the like), and relates in particular to a moving object detection method for (1) monitoring a man-made celestial object such as a small, dark man-made satellite, etc., and determining the orbit thereof, (2) discovering space debris that could cause major harm to industry if it collided with an operating man-made satellite or the like, and determining the orbit thereof, or (3) discovering at an early stage an asteroid or comet that could, in the event of a collision with Earth, have a serious impact on human industrial activities, or on the very existence of the human race, and determining the orbit thereof.
2. Description of the Related Art
FIG. 8 is a drawing to explain the conventional method for detecting a moving celestial object. The observation pictures (1-1) and (1-2) in FIG. 8 are pictures of the same celestial region in time over a certain time interval in the order of (1-1), (1-2). The objects 1 through 9 in the picture (1-1) and the objects 1′ through 9′ in the picture (1-2) are images of fixed stars, and the respective objects 1 and 1′, 2 and 2′ and so on up to 9 and 9′ represent the same fixed stars. The object 10 in picture (1-1) and 10′ in picture (1-2) represent a moving celestial object, and it can be seen that such object has moved in the sky during the image capture interval.
In the conventional moving celestial object detection method, two pictures obtained through image capture are analyzed, and the celestial objects that are reproduced in both pictures are identified and catalogued (see the catalogs (1-1D) and (1-2D) in FIG. 8). The catalogs contain such information as the position and brightness (‘Mag’) of the celestial objects 1 through 10 and 1′ through 10′, respectively. By comparing these two catalogs and detecting a pair of celestial objects that do not exist in the same position in the two pictures, the moving celestial object 10, 10′ can be determined to exist.
Incidentally, in recent years, as CCDs have become larger, the amount of sky that can be observed in one image capture session has increased dramatically. As a result, while the probability of locating a moving celestial object has increased, the number of fixed stars, which amount to noise insofar as moving celestial object detection is concerned, has also increased, and now number in the several tens of thousands. Detecting moving celestial objects in the midst of this large background of noise is extremely difficult. Moreover, because a moving celestial object moves in the pictures, sufficient results cannot be obtained when a dark moving celestial object is to be detected, even when using a fixed star observation technology in which the exposure period is lengthened so that the light from a celestial object is accumulated at a fixed position in the picture.
The effective exposure period is the period during which the moving celestial object remains at one point on the image, and the limiting magnitude is determined by the diameter of the telescope used for observation and the quantum efficiency of the CCD.
The inventors have already suggested a method that removes the images of large bright light sources that hinder the detection of a moving object which moves in any direction and at any speed and can be detected with an observation system that has been used, by using a plurality of pictures and enables detection of a moving object that is too dark to be detected with a single observation picture. With this method, a moving object that is too dark to be detected could apparently be detected by eliminating the effect of bright light sources such as fixed stars and suppressing the background noise by means of calculating the median values of each pixel value of cut-out pictures that were cut out from a plurality of images.
However, because the bright light sources (fixed stars) have a spread in the form of a central portion and peripheral portion thereof on the pictures and there is also a loss of pixels which do not return the pixel values and pictures of dark fixed stars, images of those stars produce a significant effect on calculation of median values of pixel values of cut-out pictures. Accordingly, there is still room for improvement in terms of reducing the effect of those stars and accurately and effectively detecting the dark moving bodies.